Thick film dielectric electroluminescent devices as exemplified by U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference) are typically fabricated on ceramic or glass substrates and provide superior resistance to dielectric breakdown, as well as a reduced operating voltage compared to thin film electroluminescent (TFEL) displays fabricated on glass substrates. The thick film dielectric layer of these devices have a high dielectric constant and provide a uniform pinhole-free insulating layer enabling reliable high electric field excitation of the visible-light emitting, red, green, and blue phosphor layers.
The Applicant has developed various methods for the deposition of phosphors used in thick film dielectric electroluminescent devices. For example, PCT CA01/01823. discloses a method, preferably using electron beam evaporation, for the deposition of a ternary, quaternary or similar phosphor composition, in which components of the composition are located on different sources. In particular, the compositions comprise thoaluminates, thiogallates or thioindates of elements from Group IIA and Group IIB of the Periodic Table of Elements, formed from these different sources. PCT CA01/01234 discloses a dual source phosphor deposition method using dual source electron beam deposition. The various compounds of the first and second sources are provided in the ratios required to provide the required composition of the phosphor. The deposited phosphors are preferably blue emitting europium activated barium thioaluminate.
The deposition of phosphor films comprising thioaluminates, thiogallates, or thioindates is a complex process. The properties of the deposited phosphor films are sensitive to background concentrations of sulfur and oxygen species present in the deposition atmosphere that may detrimentally affect film performance. Currently there is no reliable method to introduce hydrogen sulfide as a sulfur source for phosphor deposition at sufficiently high concentrations and sufficiently high reactive temperatures and energies in order to effectively and reproducibly provide a high luminance phosphor with stable operating characteristics. In some conventional electron beam evaporation methods, hydrogen sulfide is introduced into the deposition chamber through a feed tube to distribute itself uniformly within the chamber in a random manner. In other conventional deposition methods, the substrate is bombared with hydrogen sulfide during deposition of the phosphor. However, these methods of hydrogen sulfide introduction do not fully overcome the tendency of the deposited phosphor to be unstable during operation.
There is therefore a need to provide a method for the deposition of a phosphor film in an electroluminescent display that introduces a sulfur source in a manner that obviates the problems associated with the prior art methods.